Photocatalysis involves the use of light to overcome thermodynamic and kinetic reaction barriers in chemical reactions. The energy change associated with a given chemical reaction may be described by the Gibbs Free Energy, G. In the case where the ΔG for a reaction is positive, energy must be added to the system to accomplish the desired reaction. This energy may be provided in the form of light, and photocatalysts may be used to facilitate the conversion of photons into stored chemical energy.
A non-limiting application of this concept is the capture, conversion, and storage of solar energy through the rearrangement of chemical bonds to make fuel. Solar energy can be considered to be a carbon-neutral energy source of sufficient scale to meet future global energy demand. Thus, the conversion of sunlight into chemical fuels offers a viable mechanism for renewable energy storage and utilization. A typical photocatalytic system employs at least one photoactive composition, which, upon exposure to sunlight, produces electron/hole pairs that may be used to drive chemical reactions that store energy.
In this context, several energy storing reactions are particularly suitable, including the conversion of water to hydrogen and oxygen (i.e., “water splitting”), the conversion of nitrogen to ammonia, the conversion of hydrohalic acids to hydrogen and halide (e.g. 2HBr->H2+Br2), and the conversion of carbon dioxide to any number hydrocarbon fuel products. Production of materials that may be used as fuels (e.g., the foregoing materials) is considered especially suitable; reduction of metals (e.g., Ag+ to Ag) is somewhat less suitable.
However, light does not readily cause each of these chemical reactions to occur and catalysts are needed to effect the overall transformation with efficiencies suitable for practical applications. Photochemical systems comprised of photocatalysts that may be suspended or dissolved in solution offer a low-cost mechanism solar energy conversion. Traditional approaches to photocatalysis have relied on the use of expensive materials and/or systems that operate with low efficiency. Therefore, a need remains for the development of novel photocatalysts comprised of low-cost materials that operate with high efficiency for solar energy conversion.